Stopping a leaking exhaust manifold from entering the cabin air

I have an '11 Vic that's leaking exhaust right where the exhaust manifold connects and the problem is it's getting into the cabin by way of the air heat / vent system. When the cars moving I get no exhaust in the vents at all, only happens when I'm sitting still. For the time being (cold weather), instead of fixing the exhaust I'd just like to block the outside cabin air intake. Here's a pic taken through the outside air vents on the pass side right above the blower motor..

Is the wire mesh opening seen under the vent fins the only exterior intake for the cabin air? My best idea is to cut some kind of hard plastic to match the shape and attach it with plastic epoxy resin. Exhaust smell in the cabin is fairly strong. Are there any other openings into the vent system I should be concerned about?

EDIT:

I've been told elsewhere I shouldn't do this because that exterior intake is the only way air gets into the car from the heater and AC coils. Is this true? There is an air re-circulation intake vent in the pass floorboard. I was just planning on wiring that permanently open.

Just finished and didn't run into any hitches. For some reason the internal intake door wasn't opening at all so after disconnecting and wiring it open the heat got much better. It can get super hot now. There's probably some more in-dash door problems in there because when it's set to floor it just blows out the vents. But now that the internal door is open the floor vents seem to be giving some air.

As for the external intake I just separated the top cowl and covered the vent with a zip lock bag and put a big sponge over it. Replacing the cowl squeezed the sponge down so it's fairly secure, should do the job and easy to remove.

Unfortunately the exhaust didn't seem to be leaking so I didn't get a chance to test things out. There's some duct work in the engine bay leading from the fan to the center of the firewall. I wonder if a break in that somehow could be pulling some of the fumes in?